This invention relates to a system for electrically treating emulsions and to a high pressure, high temperature, high voltage entrance bushing, which may be used for carrying electric current from an external power source to energized electrodes through a metal sidewall of a vessel used in the electrical treatment of an emulsion. More particularly, the invention relates to the high pressure end of such an entrance bushing.
Electric treaters have been employed for many years for resolving emulsions formed of immiscible external and internal liquid phases. Usually the external phase is oil-continuous, and the internal phase is a dispersed aqueous contaminating substance. The term "oil" includes various types of materials such as petroleum and its products, and various types of organic liquids. The internal phase is aqueous and can be a caustic or acid solution. One of the most common emulsions which is resolved in electric treaters is water-in-crude oil. For purposes of removing salt, the crude oil is mixed with a quantity of dispersed fresh water. A high voltage electrical field resolves the emulsion by coalescing the internal phase into a bulk phase (carrying removed salt) which separates by gravity from the continuous external crude oil phase. The terms resolution and coalescence are used in their general meanings to denote the agglomeration of the dispersed internal phase in the continuous external phase.
Conventional electric treaters have employed high voltage from external power sources of 11 to about 33 kilovolts applied to electrodes that create the electrical field. However, energizing potentials of other values have been used in certain applications. The spacing between the electrodes defining the electric field produces voltage gradients in the range of about 2.5 kilovolts to about 8.5 kilovolts per inch of spacing.
Electric treaters employed for resolving emulsions have relatively similar constructions. The treater generally employs a pressure-type metal vessel which contains an inlet for introducing the emulsion and outlets to remove the continuous phase and the coalesced internal phase. In addition, the vessel contains electrodes for creating the electrical field. One or more energized electrodes can be suspended within the vessel but are electrically isolated from its metal sidewalls. The electrical current, from an external power source, is carried to the energized electrodes through the metal sidewall of the vessel by an electrical insulating device which is termed an "entrance bushing". The entrance bushing has metal parts to integrally connect with the sidewall of the vessel; and also it has insulating components to pass the electrical current in electrical isolation through the metal sidewall of the vessel to the energized electrode. The entrance bushing must provide the necessary electrical interconnection and a liquid-tight seal at the metal sidewall of the vessel under the temperature-pressure environment in which the emulsion is resolved within the vessel.
Prior constructions of the entrance bushing have withstood moderate temperatures and pressures of the emulsion within the vessel while conducting high potential current to the energized electrode. Present state of the art techniques require that the insulating components of the entrance bushing are formed of polymertype insulating plastic materials such as Teflon. Entrance bushings with these plastic materials provide exceptional service in commercial applications on electric treaters for resolving emulsions. For example, entrance bushings shown in U.S. Pat. Nos. 2,881,125, 3,085,128, 3,303,262, and 3,666,878 have provided outstanding operational life and safety characteristics. These bushings are exceptional in design in that they operate under exceptional environments where subjected simultaneously to temperatures as high as 400.degree. F. and fluid pressures up to 150 psi during normal operation of the electric treater.
Present day operation of electric treaters, of the nature employed in refineries, is placing a heavy burden upon even these entrance bushings. The temperature and pressure of the emulsion being resolved in electric treaters have steadily increased in the recent decade. Operating conditions are being approached which the plastic insulating materials of the entrance bushing cannot withstand in terms of operational life and maintenance of high safety characteristics. In one particular oil refinery, the heating, electrical field desalting and distillation process steps are arranged to conserve heat energy. For this purpose, the crude oil is heated to substantial temperatures (e.g. 375.degree. F.) upstream of the electric treater by heat exchange with the products of distillation and other petroleum thermal treating procedures. In some instances, the oil refinery could be even more efficiently operated if the crude oil before passing through the electrical treater for desalting or dehydration purposes could be heated to temperatures approaching 500.degree. F. These severe temperatures require very high pressures of 500 psi or more to maintain the crude oil within a liquid phase condition in the electric treater. The plastic insulating material, especially Teflon, has excellent mechanical and electrical properties. However, the design of an entrance bushing to withstand these exceptionally elevated temperatures and high pressures becomes a serious task so that the present outstanding operational life and safety characteristics of entrance bushings can be maintained.
Entrance bushings constructed with proper plastic insulating material can withstand very high operating pressure in the range of several thousand psi where the temperatures are relatively low, for example, ambient temperature of 60.degree.-80.degree. F. Alternatively, these bushings can withstand relatively high temperatures of up to about 500.degree. F. where the operating pressure is relatively low, for example, 15 psi differential across the plastic insulating material. In either case, the plastic insulating material can be used within an entrance bushing and safely isolate electrical potentials up to 50 kilovolts (ac-dc) for extended periods of time and in complete safety.
Designing entrance bushings incorporating plastic insulating material is very difficult when such a device must withstand simultaneously elevated operating pressures (500 psi) and temperatures (500.degree. F.). Teflon is typical of a number of high electrical resistance plastic materials which are subject to plastic flow upon increase in temperature and pressure. At elevated temperatures, these insulating materials are subject to plastic flow when subjected simultaneously to high operating pressures.
In the conventional entrance bushing, the plastic insulating material projects as a tubular member into the vessel containing the emulsion. Lower and upper fluid seals prevent the escape of emulsion from the vessel through the internal parts of the entrance bushing. For example, the upper portion of the tubular member of the plastic is carried in a metal adapter which is mounted in the sidewall of the vessel. The insulating material has a thermal expansion coefficient several times that of the metal (steel) components of the entrance bushing. Also, this insulating material in the cycling of operating temperatures, retains the structure induced at the maximum temperature. A sustantial temperature gradient established along the tubular member and across metal parts which form fluid seals creates severe longitudinal stresses to produce seal failure. Emulsion leakage into the entrance bushing results with entry into the metal conduit which contains the electrical conductor connecting to the external power source. An electrical arc may then occur which can destroy the electrical conductor, the entrance bushing, or both. Although electrical treaters carry devices to disconnect the electrical current from the energized electrode upon such an arc, substantial repairs are usually necessary to place the electric treater into operation. Thus, as the emulsion within the electric treater increases simultaneously in both pressure and temperature, designing an adequate entrance bushing which can be operated continuously in a high degree of safety becomes a difficult challenge. Thus, systems to remove the bushing from such environment may be utilized such as shown in U.S. Pat. No. 3,719,584.
Ideally, the system with the electrical treater functions such that the entrance bushing within the vessel can operate without suffering both high temperatures and high pressures.
In U.S. Pat. No. 3,926,774 to Watson and Winslow, an electrical treater system is disclosed adapted for resolving emulsions in a high temperature, high pressure environment. Such electric treater system includes a vessel for containing the emulsion while subjected to an electric field created by electrode means, including an energized electrode mounted in electrical isolation from the vessel. A high voltage bushing mounted within the sidewall of the vessel has an elongated tubular member projecting into the vessel and immersed within the emulsion. This tubular member, of a high electrical resistance, plastic insulating material, is subject to plastic flow upon increase in temperature and pressure. A conductor extends through the tubular member and connects at its vessels interior end to the energized electrode. The tubular member at its other end is carried in fluid-tight relationship in a metal adapter mounted in the sidewall of the vessel. A heat sink means is associated with the adapter for maintaining the length of the tubular member at substantially the same temperature as the emulsion. An aperture in the metal adapter passes the conductor to the exterior of the vessel. A pressure conduit extends in fluid-tight relationship from the metal adapter to an external power source having a high voltage output. A high pressure, high voltage feed-through insulator means seals the pressure conduit at the external power source. An interconnecting high voltage conductor within a dielectric liquid is carried in electrical isolation within the pressure conduit and connects between the conductor in the entrance bushing and the high voltage output of the external power source through the feed-through insulator means. A system means maintains the dielectric liquid at substantially the same pressure as the emulsion within the vessel. This system means includes a dynamic fluid barrier for preventing intermingling of the emulsion and dielectric liquid. Heat exchanger means on the pressure conduit maintain the high pressure, high voltage feed-through insulator means at substantially the same temperature at its pressure conduit and power source terminals.
In the system of U.S. Pat. No. 3,926,774, neither the high temperature entrance bushing nor the high pressure feed-through insulator is required to withstand elevated pressures and high temperatures simultaneously. In the high temperature entrance bushing, the fluid-tight seal between the tubular member and the metal adapter is exposed to only insignificant pressure differentials. The tubular member is maintained at substantially the same temperature throughout its length, and that temperature is substantially that of the emulsion in the treating vessel. The feed-through insulator adjacent the external power source withstands the full pressure of the emulsion within the vessel. However, the insulator is maintained at the same (ambient) temperature at its pressure conduit and high voltage output ends.
For example, the high temperature entrance bushing can operate under conditions which impose a temperature of 500.degree. F. while suffering practically no pressure differential between the emulsion within the treating vessel and the dielectric liquid within the pressure conduit. The high pressure feed-through insulator may contain a fluid pressure of 500 pounds per square inch while being maintained at ambient temperatures of about 80.degree. F. This separation of the temperature and pressure operating conditions at the entrance bushing and feed-through insulator permits of a system which can operate at elevated temperatures and high pressures with the same safety as if the system were operated with a single entrance bushing or insulator containing essentially zero fluid pressure at ambient temperatures. Thus, the outstanding safety and operating records of earlier entrance bushings employed in electric field treaters are maintained for operating conditions of temperature and pressure greatly in excess of those heretofore encountered in oil refineries and other installations by using the electric treater system of U.S. Pat. No. 3,926,774.
While the entrance bushing arrangement, including the feed-through insulator of U.S. Pat. No. 3,926,774, has the advantages described above, it is not sufficiently compact so that it can be shop assembled, tested, shipped and installed as a unit.
It is accordingly an object of this invention to provide a high pressure, high temperature, high voltage entrance bushing assembly which is sufficiently compact so that it can be shop assembled, tested, shipped and installed as a unit.
It is a further object of this invention to provide an improved high-pressure end for such a high pressure, high temperature, high voltage entrance bushing assembly.
It is an additional object of this invention to provide a system for resolving emulsions which include a treating vessel containing electrodes and an improved high pressure, high temperature, high voltage entrance bushing assembly.